# Standard library header <flat_set> (C++23)

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This header is part of the containers library.

### Includes
<compare> (C++20) | Three-way comparison operator support
<initializer_list> (C++11) | std::initializer_list class template
### Classes
flat_set (C++23) | adapts a container to provide a collection of unique keys, sorted by keys (class template)
flat_multiset (C++23) | adapts a container to provide a collection of keys, sorted by keys (class template)
std::uses_allocator<std::flat_set> (C++23) | specializes the std::uses_allocator type trait (class template specialization)
std::uses_allocator<std::flat_multiset> (C++23) | specializes the std::uses_allocator type trait (class template specialization)
### Functions
erase_if(std::flat_set) (C++23) | erases all elements satisfying specific criteria (function template)
erase_if(std::flat_multiset) (C++23) | erases all elements satisfying specific criteria (function template)
### Tags
sorted_uniquesorted_unique_t (C++23) | indicates that elements of a range are sorted and unique (tag)
sorted_equivalentsorted_equivalent_t (C++23) | indicates that elements of a range are sorted (uniqueness is not required) (tag)

### Synopsis

    #include <compare>
    #include <initializer_list>

    namespace std {
      // class template flat_set
      template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
      class flat_set;

      struct sorted_unique_t
      {
        explicit sorted_unique_t() = default;
      };
      inline constexpr sorted_unique_t sorted_unique{};

      template<class Key, class Compare, class KeyContainer, class Allocator>
      struct uses_allocator<flat_set<Key, Compare, KeyContainer>, Allocator>;

      // erasure for flat_set
      template<class Key, class Compare, class KeyContainer, class Predicate>
      typename flat_set<Key, Compare, KeyContainer>::size_type erase_if(
        flat_set<Key, Compare, KeyContainer>& c,
        Predicate pred);

      // class template flat_multiset
      template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
      class flat_multiset;

      struct sorted_equivalent_t
      {
        explicit sorted_equivalent_t() = default;
      };
      inline constexpr sorted_equivalent_t sorted_equivalent{};

      template<class Key, class Compare, class KeyContainer, class Allocator>
      struct uses_allocator<flat_multiset<Key, Compare, KeyContainer>, Allocator>;

      // erasure for flat_multiset
      template<class Key, class Compare, class KeyContainer, class Predicate>
      typename flat_multiset<Key, Compare, KeyContainer>::size_type erase_if(
        flat_multiset<Key, Compare, KeyContainer>& c,
        Predicate pred);
    }

#### Class template std::flat_set

    namespace std {
      template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
      class flat_set
      {
      public:
        // types
        using key_type               = Key;
        using value_type             = Key;
        using key_compare            = Compare;
        using value_compare          = Compare;
        using reference              = value_type&;
        using const_reference        = const value_type&;
        using size_type              = typename KeyContainer::size_type;
        using difference_type        = typename KeyContainer::difference_type;
        using iterator               = /* implementation-defined */;
        using const_iterator         = /* implementation-defined */;
        using reverse_iterator       = std::reverse_iterator<iterator>;
        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
        using container_type         = KeyContainer;

        // constructors
        flat_set()
          : flat_set(key_compare())
        {
        }

        explicit flat_set(const key_compare& comp)
          : /*c*/()
          , /*compare*/(comp)
        {
        }

        explicit flat_set(container_type cont, const key_compare& comp = key_compare());

        flat_set(sorted_unique_t,
                 container_type cont,
                 const key_compare& comp = key_compare())
          : /*c*/(std::move(cont))
          , /*compare*/(comp)
        {
        }

        template<class InputIter>
        flat_set(InputIter first, InputIter last, const key_compare& comp = key_compare())
          : /*c*/()
          , /*compare*/(comp)
        {
          insert(first, last);
        }

        template<class InputIter>
        flat_set(sorted_unique_t,
                 InputIter first,
                 InputIter last,
                 const key_compare& comp = key_compare())
          : /*c*/(first, last)
          , /*compare*/(comp)
        {
        }

        template<container-compatible-range<value_type> R>
        flat_set(from_range_t fr, R&& rg)
          : flat_set(fr, std::forward<R>(rg), key_compare())
        {
        }
        template<container-compatible-range<value_type> R>
        flat_set(from_range_t, R&& rg, const key_compare& comp)
          : flat_set(comp)
        {
          insert_range(std::forward<R>(rg));
        }

        flat_set(initializer_list<value_type> il, const key_compare& comp = key_compare())
          : flat_set(il.begin(), il.end(), comp)
        {
        }

        flat_set(sorted_unique_t s,
                 initializer_list<value_type> il,
                 const key_compare& comp = key_compare())
          : flat_set(s, il.begin(), il.end(), comp)
        {
        }

        // constructors with allocators

        template<class Alloc>
        explicit flat_set(const Alloc& a);
        template<class Alloc>
        flat_set(const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_set(const container_type& cont, const Alloc& a);
        template<class Alloc>
        flat_set(const container_type& cont, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_set(sorted_unique_t, const container_type& cont, const Alloc& a);
        template<class Alloc>
        flat_set(sorted_unique_t,
                 const container_type& cont,
                 const key_compare& comp,
                 const Alloc& a);
        template<class Alloc>
        flat_set(const flat_set&, const Alloc& a);
        template<class Alloc>
        flat_set(flat_set&&, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_set(InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_set(InputIter first, InputIter last, const key_compare& comp, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_set(sorted_unique_t, InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_set(sorted_unique_t,
                 InputIter first,
                 InputIter last,
                 const key_compare& comp,
                 const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_set(from_range_t, R&& rg, const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_set(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_set(initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_set(initializer_list<value_type> il, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_set(sorted_unique_t, initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_set(sorted_unique_t,
                 initializer_list<value_type> il,
                 const key_compare& comp,
                 const Alloc& a);

        flat_set& operator=(initializer_list<value_type>);

        // iterators
        iterator begin() noexcept;
        const_iterator begin() const noexcept;
        iterator end() noexcept;
        const_iterator end() const noexcept;

        reverse_iterator rbegin() noexcept;
        const_reverse_iterator rbegin() const noexcept;
        reverse_iterator rend() noexcept;
        const_reverse_iterator rend() const noexcept;

        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;
        const_reverse_iterator crbegin() const noexcept;
        const_reverse_iterator crend() const noexcept;

        // capacity
        [[nodiscard]] bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // modifiers
        template<class... Args>
        pair<iterator, bool> emplace(Args&&... args);
        template<class... Args>
        iterator emplace_hint(const_iterator position, Args&&... args);

        pair<iterator, bool> insert(const value_type& x) { return emplace(x); }
        pair<iterator, bool> insert(value_type&& x) { return emplace(std::move(x)); }
        template<class K>
        pair<iterator, bool> insert(K&& x);
        iterator insert(const_iterator position, const value_type& x)
        {
          return emplace_hint(position, x);
        }
        iterator insert(const_iterator position, value_type&& x)
        {
          return emplace_hint(position, std::move(x));
        }
        template<class K>
        iterator insert(const_iterator hint, K&& x);

        template<class InputIter>
        void insert(InputIter first, InputIter last);
        template<class InputIter>
        void insert(sorted_unique_t, InputIter first, InputIter last);
        template<container-compatible-range<value_type> R>
        void insert_range(R&& rg);

        void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
        void insert(sorted_unique_t s, initializer_list<value_type> il)
        {
          insert(s, il.begin(), il.end());
        }

        container_type extract() &&;
        void replace(container_type&&);

        iterator erase(iterator position);
        iterator erase(const_iterator position);
        size_type erase(const key_type& x);
        template<class K>
        size_type erase(K&& x);
        iterator erase(const_iterator first, const_iterator last);

        void swap(flat_set& y) noexcept;
        void clear() noexcept;

        // observers
        key_compare key_comp() const;
        value_compare value_comp() const;

        // set operations
        iterator find(const key_type& x);
        const_iterator find(const key_type& x) const;
        template<class K>
        iterator find(const K& x);
        template<class K>
        const_iterator find(const K& x) const;

        size_type count(const key_type& x) const;
        template<class K>
        size_type count(const K& x) const;

        bool contains(const key_type& x) const;
        template<class K>
        bool contains(const K& x) const;

        iterator lower_bound(const key_type& x);
        const_iterator lower_bound(const key_type& x) const;
        template<class K>
        iterator lower_bound(const K& x);
        template<class K>
        const_iterator lower_bound(const K& x) const;

        iterator upper_bound(const key_type& x);
        const_iterator upper_bound(const key_type& x) const;
        template<class K>
        iterator upper_bound(const K& x);
        template<class K>
        const_iterator upper_bound(const K& x) const;

        pair<iterator, iterator> equal_range(const key_type& x);
        pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
        template<class K>
        pair<iterator, iterator> equal_range(const K& x);
        template<class K>
        pair<const_iterator, const_iterator> equal_range(const K& x) const;

        friend bool operator==(const flat_set& x, const flat_set& y);

        friend /*synth-three-way-result*/<value_type> operator<=>(const flat_set& x,
                                                                  const flat_set& y);

        friend void swap(flat_set& x, flat_set& y) noexcept { x.swap(y); }

      private:
        container_type /*c*/;    // exposition only
        key_compare /*compare*/; // exposition only
      };

      template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
      flat_set(KeyContainer, Compare = Compare())
        -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;
      template<class KeyContainer, class Allocator>
      flat_set(KeyContainer, Allocator) -> flat_set<typename KeyContainer::value_type,
                                                    less<typename KeyContainer::value_type>,
                                                    KeyContainer>;
      template<class KeyContainer, class Compare, class Allocator>
      flat_set(KeyContainer, Compare, Allocator)
        -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;

      template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
      flat_set(sorted_unique_t, KeyContainer, Compare = Compare())
        -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;
      template<class KeyContainer, class Allocator>
      flat_set(sorted_unique_t, KeyContainer, Allocator)
        -> flat_set<typename KeyContainer::value_type,
                    less<typename KeyContainer::value_type>,
                    KeyContainer>;
      template<class KeyContainer, class Compare, class Allocator>
      flat_set(sorted_unique_t, KeyContainer, Compare, Allocator)
        -> flat_set<typename KeyContainer::value_type, Compare, KeyContainer>;

      template<class InputIter, class Compare = less</*iter-value-type*/<InputIter>>>
      flat_set(InputIter, InputIter, Compare = Compare())
        -> flat_set</*iter-value-type*/<InputIter>, Compare>;

      template<class InputIter, class Compare = less</*iter-value-type*/<InputIter>>>
      flat_set(sorted_unique_t, InputIter, InputIter, Compare = Compare())
        -> flat_set</*iter-value-type*/<InputIter>, Compare>;

      template<ranges::input_range R,
               class Compare   = less<ranges::range_value_t<R>>,
               class Allocator = allocator<ranges::range_value_t<R>>>
      flat_set(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
        -> flat_set<ranges::range_value_t<R>,
                    Compare,
                    vector<ranges::range_value_t<R>,
                           /*alloc-rebind*/<Allocator, ranges::range_value_t<R>>>>;

      template<ranges::input_range R, class Allocator>
      flat_set(from_range_t, R&&, Allocator)
        -> flat_set<ranges::range_value_t<R>,
                    less<ranges::range_value_t<R>>,
                    vector<ranges::range_value_t<R>,
                           /*alloc-rebind*/<Allocator, ranges::range_value_t<R>>>>;

      template<class Key, class Compare = less<Key>>
      flat_set(initializer_list<Key>, Compare = Compare()) -> flat_set<Key, Compare>;

      template<class Key, class Compare = less<Key>>
      flat_set(sorted_unique_t, initializer_list<Key>, Compare = Compare())
        -> flat_set<Key, Compare>;

      template<class Key, class Compare, class KeyContainer, class Allocator>
      struct uses_allocator<flat_set<Key, Compare, KeyContainer>, Allocator>
        : bool_constant<uses_allocator_v<KeyContainer, Allocator>>
      {};
    }

#### Class template std::flat_multiset

    namespace std {
      template<class Key, class Compare = less<Key>, class KeyContainer = vector<Key>>
      class flat_multiset
      {
      public:
        // types
        using key_type               = Key;
        using value_type             = Key;
        using key_compare            = Compare;
        using value_compare          = Compare;
        using reference              = value_type&;
        using const_reference        = const value_type&;
        using size_type              = typename KeyContainer::size_type;
        using difference_type        = typename KeyContainer::difference_type;
        using iterator               = /* implementation-defined */;
        using const_iterator         = /* implementation-defined */;
        using reverse_iterator       = std::reverse_iterator<iterator>;
        using const_reverse_iterator = std::reverse_iterator<const_iterator>;
        using container_type         = KeyContainer;

        // constructors
        flat_multiset()
          : flat_multiset(key_compare())
        {
        }

        explicit flat_multiset(const key_compare& comp)
          : /*c*/()
          , /*compare*/(comp)
        {
        }

        explicit flat_multiset(container_type cont, const key_compare& comp = key_compare());

        flat_multiset(sorted_equivalent_t,
                      container_type cont,
                      const key_compare& comp = key_compare())
          : /*c*/(std::move(cont))
          , /*compare*/(comp)
        {
        }

        template<class InputIter>
        flat_multiset(InputIter first,
                      InputIter last,
                      const key_compare& comp = key_compare())
          : /*c*/()
          , /*compare*/(comp)
        {
          insert(first, last);
        }

        template<class InputIter>
        flat_multiset(sorted_equivalent_t,
                      InputIter first,
                      InputIter last,
                      const key_compare& comp = key_compare())
          : /*c*/(first, last)
          , /*compare*/(comp)
        {
        }

        template<container-compatible-range<value_type> R>
        flat_multiset(from_range_t fr, R&& rg)
          : flat_multiset(fr, std::forward<R>(rg), key_compare())
        {
        }
        template<container-compatible-range<value_type> R>
        flat_multiset(from_range_t, R&& rg, const key_compare& comp)
          : flat_multiset(comp)
        {
          insert_range(std::forward<R>(rg));
        }

        flat_multiset(initializer_list<value_type> il,
                      const key_compare& comp = key_compare())
          : flat_multiset(il.begin(), il.end(), comp)
        {
        }

        flat_multiset(sorted_equivalent_t s,
                      initializer_list<value_type> il,
                      const key_compare& comp = key_compare())
          : flat_multiset(s, il.begin(), il.end(), comp)
        {
        }

        // constructors with allocators

        template<class Alloc>
        explicit flat_multiset(const Alloc& a);
        template<class Alloc>
        flat_multiset(const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_multiset(const container_type& cont, const Alloc& a);
        template<class Alloc>
        flat_multiset(const container_type& cont, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_multiset(sorted_equivalent_t, const container_type& cont, const Alloc& a);
        template<class Alloc>
        flat_multiset(sorted_equivalent_t,
                      const container_type& cont,
                      const key_compare& comp,
                      const Alloc& a);
        template<class Alloc>
        flat_multiset(const flat_multiset&, const Alloc& a);
        template<class Alloc>
        flat_multiset(flat_multiset&&, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multiset(InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multiset(InputIter first,
                      InputIter last,
                      const key_compare& comp,
                      const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multiset(sorted_equivalent_t, InputIter first, InputIter last, const Alloc& a);
        template<class InputIter, class Alloc>
        flat_multiset(sorted_equivalent_t,
                      InputIter first,
                      InputIter last,
                      const key_compare& comp,
                      const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_multiset(from_range_t, R&& rg, const Alloc& a);
        template<container-compatible-range<value_type> R, class Alloc>
        flat_multiset(from_range_t, R&& rg, const key_compare& comp, const Alloc& a);
        template<class Alloc>
        flat_multiset(initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_multiset(initializer_list<value_type> il,
                      const key_compare& comp,
                      const Alloc& a);
        template<class Alloc>
        flat_multiset(sorted_equivalent_t, initializer_list<value_type> il, const Alloc& a);
        template<class Alloc>
        flat_multiset(sorted_equivalent_t,
                      initializer_list<value_type> il,
                      const key_compare& comp,
                      const Alloc& a);

        flat_multiset& operator=(initializer_list<value_type>);

        // iterators
        iterator begin() noexcept;
        const_iterator begin() const noexcept;
        iterator end() noexcept;
        const_iterator end() const noexcept;

        reverse_iterator rbegin() noexcept;
        const_reverse_iterator rbegin() const noexcept;
        reverse_iterator rend() noexcept;
        const_reverse_iterator rend() const noexcept;

        const_iterator cbegin() const noexcept;
        const_iterator cend() const noexcept;
        const_reverse_iterator crbegin() const noexcept;
        const_reverse_iterator crend() const noexcept;

        // capacity
        [[nodiscard]] bool empty() const noexcept;
        size_type size() const noexcept;
        size_type max_size() const noexcept;

        // modifiers
        template<class... Args>
        iterator emplace(Args&&... args);
        template<class... Args>
        iterator emplace_hint(const_iterator position, Args&&... args);

        iterator insert(const value_type& x) { return emplace(x); }
        iterator insert(value_type&& x) { return emplace(std::move(x)); }
        iterator insert(const_iterator position, const value_type& x)
        {
          return emplace_hint(position, x);
        }
        iterator insert(const_iterator position, value_type&& x)
        {
          return emplace_hint(position, std::move(x));
        }

        template<class InputIter>
        void insert(InputIter first, InputIter last);
        template<class InputIter>
        void insert(sorted_equivalent_t, InputIter first, InputIter last);
        template<container-compatible-range<value_type> R>
        void insert_range(R&& rg);

        void insert(initializer_list<value_type> il) { insert(il.begin(), il.end()); }
        void insert(sorted_equivalent_t s, initializer_list<value_type> il)
        {
          insert(s, il.begin(), il.end());
        }

        container_type extract() &&;
        void replace(container_type&&);

        iterator erase(iterator position);
        iterator erase(const_iterator position);
        size_type erase(const key_type& x);
        template<class K>
        size_type erase(K&& x);
        iterator erase(const_iterator first, const_iterator last);

        void swap(flat_multiset& y) noexcept;
        void clear() noexcept;

        // observers
        key_compare key_comp() const;
        value_compare value_comp() const;

        // set operations
        iterator find(const key_type& x);
        const_iterator find(const key_type& x) const;
        template<class K>
        iterator find(const K& x);
        template<class K>
        const_iterator find(const K& x) const;

        size_type count(const key_type& x) const;
        template<class K>
        size_type count(const K& x) const;

        bool contains(const key_type& x) const;
        template<class K>
        bool contains(const K& x) const;

        iterator lower_bound(const key_type& x);
        const_iterator lower_bound(const key_type& x) const;
        template<class K>
        iterator lower_bound(const K& x);
        template<class K>
        const_iterator lower_bound(const K& x) const;

        iterator upper_bound(const key_type& x);
        const_iterator upper_bound(const key_type& x) const;
        template<class K>
        iterator upper_bound(const K& x);
        template<class K>
        const_iterator upper_bound(const K& x) const;

        pair<iterator, iterator> equal_range(const key_type& x);
        pair<const_iterator, const_iterator> equal_range(const key_type& x) const;
        template<class K>
        pair<iterator, iterator> equal_range(const K& x);
        template<class K>
        pair<const_iterator, const_iterator> equal_range(const K& x) const;

        friend bool operator==(const flat_multiset& x, const flat_multiset& y);

        friend /*synth-three-way-result*/<value_type> operator<=>(const flat_multiset& x,
                                                                  const flat_multiset& y);

        friend void swap(flat_multiset& x, flat_multiset& y) noexcept { x.swap(y); }

      private:
        container_type /*c*/;    // exposition only
        key_compare /*compare*/; // exposition only
      };

      template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
      flat_multiset(KeyContainer, Compare = Compare())
        -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;
      template<class KeyContainer, class Allocator>
      flat_multiset(KeyContainer, Allocator)
        -> flat_multiset<typename KeyContainer::value_type,
                         less<typename KeyContainer::value_type>,
                         KeyContainer>;
      template<class KeyContainer, class Compare, class Allocator>
      flat_multiset(KeyContainer, Compare, Allocator)
        -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;

      template<class KeyContainer, class Compare = less<typename KeyContainer::value_type>>
      flat_multiset(sorted_equivalent_t, KeyContainer, Compare = Compare())
        -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;
      template<class KeyContainer, class Allocator>
      flat_multiset(sorted_equivalent_t, KeyContainer, Allocator)
        -> flat_multiset<typename KeyContainer::value_type,
                         less<typename KeyContainer::value_type>,
                         KeyContainer>;
      template<class KeyContainer, class Compare, class Allocator>
      flat_multiset(sorted_equivalent_t, KeyContainer, Compare, Allocator)
        -> flat_multiset<typename KeyContainer::value_type, Compare, KeyContainer>;

      template<class InputIter, class Compare = less</*iter-value-type*/<InputIter>>>
      flat_multiset(InputIter, InputIter, Compare = Compare())
        -> flat_multiset</*iter-value-type*/<InputIter>, Compare>;

      template<class InputIter, class Compare = less</*iter-value-type*/<InputIter>>>
      flat_multiset(sorted_equivalent_t, InputIter, InputIter, Compare = Compare())
        -> flat_multiset</*iter-value-type*/<InputIter>, Compare>;

      template<ranges::input_range R,
               class Compare   = less<ranges::range_value_t<R>>,
               class Allocator = allocator<ranges::range_value_t<R>>>
      flat_multiset(from_range_t, R&&, Compare = Compare(), Allocator = Allocator())
        -> flat_multiset<ranges::range_value_t<R>,
                         Compare,
                         vector<ranges::range_value_t<R>,
                                /*alloc-rebind*/<Allocator, ranges::range_value_t<R>>>>;

      template<ranges::input_range R, class Allocator>
      flat_multiset(from_range_t, R&&, Allocator)
        -> flat_multiset<ranges::range_value_t<R>,
                         less<ranges::range_value_t<R>>,
                         vector<ranges::range_value_t<R>,
                                /*alloc-rebind*/<Allocator, ranges::range_value_t<R>>>>;

      template<class Key, class Compare = less<Key>>
      flat_multiset(initializer_list<Key>, Compare = Compare())
        -> flat_multiset<Key, Compare>;

      template<class Key, class Compare = less<Key>>
      flat_multiset(sorted_equivalent_t, initializer_list<Key>, Compare = Compare())
        -> flat_multiset<Key, Compare>;

      template<class Key, class Compare, class KeyContainer, class Allocator>
      struct uses_allocator<flat_multiset<Key, Compare, KeyContainer>, Allocator>
        : bool_constant<uses_allocator_v<KeyContainer, Allocator>>
      {};
    }

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 24.6.5 Header `<flat_set>` synopsis [flat.set.syn]

  - 24.6.11.2 Definition [flat.set.defn]

  - 24.6.12.2 Definition [flat.multiset.defn]

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